1. Field of the Invention
The present invention relates generally to the field of processing information contained on coded tags, and more particularly, to a threshold detector for use in an optical character recognition system.
2. Description of the Prior Art
In many enterprises, such as grocery stores, post offices, parts supply warehouses, etc., large numbers of items must be handled during a specified time period. For example, a clerk at a check out counter in a grocery store must properly handle each item that is purchased so that the customer is correctly billed and/or the item is correctly inventoried. In the past such handling has been accomplished by manually ringing up the required amount on a cash register. However, recently with the advent of point-of-sale systems, this handling is able to be done automatically with the use of coded tags, labels or the like, that are fastened to the items and read by an appropriate reader device. A conventional tag comprises a front surface having black characters, such as letters, numbers, marks, etc., on a white background.
One type of prior art device for sensing the characters employs a fixed threshold detector circuit. The device serves to provide an appropriate signal when the blackness associated with the character exceeds a reference level. However, such a device has been found to be undesirable since variations in the parameters affecting the threshold level must be either tolerated or be capable of normalization by adjusting one or more of its components. For example, adjustment is typically required of the gain of a linear amplifier, the intensity of a light source or of a plurality of individual light sources, the ambient temperature, etc. The drawbacks of the described approaches reside not only in the individual adjustments which must be made, but in the fact that subsequent readjustments are periodically required to accommodate degradation of the light source and/or the photo sensing device and its associated circuit components. In addition, since ambient conditions such as dust and other airborne deposits tend to compromise the associated optical system, and consequently the signal strength provided by the photo sensing device, regular maintenance of the device is required.
Furthermore, it should be noted that one of the practical difficulties in systems that serve to optically scan tags having characters or marks on them is the fact that the density, opacity, blackness or readability of the characters is extremely variable due to the wide fluctuations in the quality of the printing ink used. This causes variations in the signal level of an analog video signal corresponding in part to the black character. Moreover, due to a varying reflectivity characteristic of the white surface being scanned, the degradation with age of the light source, and the varying characteristics of the photo sensors, the "white" level of the video signal is subject to simultaneous variation. Because of the simultaneous variations in the black and white levels, fixed threshold detector circuits are subject to errors in recognizing characters.
Another type of device for sensing optical marks is found in U.S. Pat. No. 3,560,751, entitled, "Optical Mark Sensing Device," by Donald L. Buettner et al. The patent teaches a complex device that includes a capacitive means for storing a voltage level representative of background illumination and that emits a signal upon the occurrence of a proportional reduction of light level indicative of a mark in the sensing area.
Yet another type of prior art device is found in U.S. Pat. No. 3,820,068, entitled, "Background Reference Level System and Method for Document Scanners," by John V. McMillin. The disclosed system is adapted to check marks in a multiplicity of data channels and consequently includes a relatively complex analog-to-digital converter.